The background for the present invention is drawn from two distinct technologies. One technology addresses the response of structures and structural members to loading, and design technologies directed toward preventing failures of such members during service. The other technology addresses the manufacture of structural members (and components thereof) such as those related to the present invention.
Engineers who design various structures typically consider many potential failure modes during the design process. They also consider the types of loading, and the environment, in which a structure will operate. They examine the possibility of overload, wherein a material employed in the structure is loaded beyond its strength capability. In this situation, the mode of failure may be brittle, where failure occurs with minimal deformation, or ductile, where some deformation of the material occurs prior to fracture. Ductile failure comprises permanent or plastic deformation, typically of sufficient magnitude such that the structural member assumes a new shape that is unable to perform the intended function of that member.
Engineers also consider the effects of an aggressive environment on a material, wherein the environment degrades the strength of the material. They consider the effects of temperature on a material, for the ability of a material to withstand a load typically decreases with increasing temperature. They consider cyclic loading, wherein a crack can develop and grow to the point where catastrophic failure occurs. These failure modes may occur instantly, or gradually over the intended service life of the structural member.
Engineers must also consider the possibility that the configuration of the structure may become unstable in service, and if so, failure may consequently occur in a mode called buckling. One situation where buckling is a major design consideration is a slender member that is subjected to axial compressive loading. An example of this situation exists in many types of buildings, where a column is employed to support the floors and walls of an upper level of the building. To conserve building materials, and to maximize flexibility of building design, such columns are desirably as small in cross section as is safe. The problem of column buckling has existed since prehistoric man created the first towers and buildings. Leonhard Euler, the eminent Swiss mathematician and physicist, published a solution to the column buckling problem in 1744. He described a critical load necessary to cause buckling as a function of the physical dimensions of the structural member, notably its length and cross sectional configuration, and the modulus of elasticity of the material employed in that member. It should be noted that the strength of the material employed in the column is not a factor in Euler's solution to the column buckling problem.
It has also been shown that the critical load necessary to cause buckling of a column also depends on the manner of application of service loads to the structural member and constraints against movement of the extremities of the structural member.
There are many other types of structural members that are subject to buckling. Some of these are: beams in three-point bending, flat sheets subjected to in-plane bending, and shafts in torsion. In the latter case, buckling may occur by twisting a straight structural member into a coil-like shape, or by collapsing a tubular structural member across its own cross section. Tubular structural members are very useful because they are lighter than solid members of similar shape, but they are particularly vulnerable to buckling. Solutions to these and other buckling problems are well known to those who are skilled in the applicable arts.
As a practical matter, the design of structures and structural members that may be subject to buckling is focused on that mode of failure. Once an engineer has designed a structure to withstand the intended service loads without buckling, he/she then checks the design to confirm that the structure can withstand such service loads without failing in other modes of failure. With such a design procedure, a column or shaft designed to prevent buckling will generally be tubular in overall configuration. To facilitate both design and manufacture, such structural members are often circular in cross section. The structural member must have a diameter large enough so that the member maintains its overall straight configuration, and a wall thickness great enough to eliminate change in cross sectional shape as a cause of buckling.
One result of this design procedure is that it leads to wall thicknesses that are greater than might be desired, particularly from the perspective of minimizing weight of structural components. The present invention addresses this matter, comprising a novel approach to designing and fabricating lightweight structural components that are highly resistant to buckling.
One embodiment of the present invention comprises placement of metallic foam in an internal cavity in a hollow structural member, thereby inhibiting the buckling of the structural member. There are many techniques for producing metallic foams; Knott et al (U.S. Pat. No. 6,444,007) list five categories of procedures for manufacturing metallic foams. Other techniques and variations in the procedures listed by Knott et al are possible. Some of these techniques are relevant to the present invention, provided that problems involving placement of foam within the structural member can be solved. The disclosures of Jin et al (U.S. Pat. No. 4,973,358), Kenny et al (U.S. Pat. No. 5,281,251) and Sang et al (U.S. Pat. No. 5,334,236), all assigned to a common assignee, describe technologies for producing metallic foam that can be configured to a predetermined shape, subject to certain process limitations. Shapovalov (U.S. Pat. No. 5,181,549) discloses another method for producing metallic foam. He further discloses a method for producing metallic foam having a skin of the same material as the foam itself. However, there are significant limitations in the shapes of products that can be made by Shapovalov's methods. In the context of the present invention, these, and other such methods, may be employed for placing the foam in an internal cavity in a structural member. The methods described hereinabove are exemplary of prior art. One skilled in the applicable arts will recognize that these methods may be adapted to the present invention without limiting the true scope and breadth of the present invention.